15213 Recitation Section C

1. Shimin Chen Nov. 11, 2002 15213 Recitation Section C Understanding mm-helper.c Adding debugging info to mm-helper.c Outline

2. What does mm-helper.c do ? • Implicit Free List • Header with each block – (size / allocated bit) • No separate Free List – free blocks linked implicitly by size fields in header • First Fit • Searches free list from beginning and picks first block that is large enough • Immediate Boundary Tag Coalescing • Footer (boundary tag), replica of header

3. Block Format Header 32 bits Pointer returned by malloc (Block Pointer (bp)) Payload >= what user asked for in malloc Footer 32 bits

4. 3 2 1 0 31 size 0 0 a Header/Footer Format • Double word alignment • Three lower-order bits of size always 0 • Pack size and allocated bits into a single integer • Size = 24 (0x18). Block is allocated Header = 0 x18 | 0x1 = 0x19

5. Footer Header Header Footer Double Word Alignment (8 bytes) Heap Format 4 bytes Pad Prologue Epilogue 8|1 8|1 0|1 Allocated and Free Blocks

6. Very Useful Macros • #define WSIZE 4 • #define DSIZE 8 • #define CHUNKSIZE (1<<12) • #define OVERHEAD 8

7. Very Useful Macros • #define PACK(size, alloc) ((size) | (alloc)) • #define GET(p) (*(size_t *)(p)) • #define PUT(p, val) (*(size_t *)(p) = (val)) • #define GET_SIZE(p) (GET(p) & ~0x7) • #define GET_ALLOC(p) (GET(p) & 0x1)

8. Very Useful Macros • #define HDRP(bp) ((char *)(bp) - WSIZE) • #define FTRP(bp) ((char *)(bp) + GET_SIZE(HDRP(bp)) - DSIZE) • #define NEXT_BLKP(bp) ((char *)(bp) + GET_SIZE(((char *)(bp) - WSIZE))) • #define PREV_BLKP(bp) ((char *)(bp) - GET_SIZE(((char *)(bp) - DSIZE)))

9. Initializing the Heap int mm_init(void) { if ((heap_listp = mem_sbrk(4*WSIZE)) == NULL) return -1; PUT(heap_listp, 0); PUT(heap_listp+WSIZE, PACK(OVERHEAD, 1)); PUT(heap_listp+DSIZE, PACK(OVERHEAD, 1)); PUT(heap_listp+WSIZE+DSIZE, PACK(0, 1)); heap_listp += DSIZE; if (extend_heap(CHUNKSIZE/WSIZE) == NULL) return -1; return 0; }

10. Extending the Heap static void *extend_heap(size_t words) { char *bp; size_t size; size = (words % 2) ? (words+1)*WSIZE : words*WSIZE; if ((int)(bp = mem_sbrk(size)) < 0) return NULL; PUT(HDRP(bp), PACK(size, 0)); PUT(FTRP(bp), PACK(size, 0)); PUT(HDRP(NEXT_BLKP(bp)), PACK(0, 1)); return coalesce(bp); }

11. Malloc void *mm_malloc(size_t size) { size_t asize, extendsize; char *bp; if (size <= 0) return NULL; if (size <= DSIZE) asize = DSIZE+OVERHEAD; else asize = DSIZE*((size+(OVERHEAD)+(DSIZE-1))/DSIZE); if ((bp = find_fit(asize)) != NULL) { place(bp, asize); return bp; } extendsize = MAX(asize,CHUNKSIZE); if ((bp = extend_heap(extendsize/WSIZE)) == NULL) return NULL; place(bp, asize); return bp; }

12. Finding First Fit static void *find_fit(size_t asize) { void *bp; for (bp = heap_listp; GET_SIZE(HDRP(bp)) > 0; bp = NEXT_BLKP(bp)) if (!GET_ALLOC(HDRP(bp)) && (asize <= GET_SIZE(HDRP(bp)))) return bp; return NULL; }

13. Malloc void *mm_malloc(size_t size) { size_t asize, extendsize; char *bp; if (size <= 0) return NULL; if (size <= DSIZE) asize = DSIZE+OVERHEAD; else asize = DSIZE*((size+(OVERHEAD)+(DSIZE-1))/DSIZE); if ((bp = find_fit(asize)) != NULL) { place(bp, asize); return bp; } extendsize = MAX(asize,CHUNKSIZE); if ((bp = extend_heap(extendsize/WSIZE)) == NULL) return NULL; place(bp, asize); return bp; }

14. Placing a Block in a Free Block static void place(void *bp, size_t asize) { size_t csize = GET_SIZE(HDRP(bp)); if ((csize - asize) >= (DSIZE + OVERHEAD)) { PUT(HDRP(bp), PACK(asize, 1)); PUT(FTRP(bp), PACK(asize, 1)); bp = NEXT_BLKP(bp); PUT(HDRP(bp), PACK(csize-asize, 0)); PUT(FTRP(bp), PACK(csize-asize, 0)); } else { PUT(HDRP(bp), PACK(csize, 1)); PUT(FTRP(bp), PACK(csize, 1)); } }

15. Malloc void *mm_malloc(size_t size) { size_t asize, extendsize; char *bp; if (size <= 0) return NULL; if (size <= DSIZE) asize = DSIZE+OVERHEAD; else asize = DSIZE*((size+(OVERHEAD)+(DSIZE-1))/DSIZE); if ((bp = find_fit(asize)) != NULL) { place(bp, asize); return bp; } extendsize = MAX(asize,CHUNKSIZE); if ((bp = extend_heap(extendsize/WSIZE)) == NULL) return NULL; place(bp, asize); return bp; }

16. Free void mm_free(void *bp) { size_t size = GET_SIZE(HDRP(bp)); PUT(HDRP(bp), PACK(size, 0)); PUT(FTRP(bp), PACK(size, 0)); coalesce(bp); }

17. Coalesce: Called by mm_free() & extend_heap() static void *coalesce(void *bp) { size_t prev_alloc = GET_ALLOC(FTRP(PREV_BLKP(bp))); size_t next_alloc = GET_ALLOC(HDRP(NEXT_BLKP(bp))); size_t size = GET_SIZE(HDRP(bp)); if (prev_alloc && next_alloc) { return bp; } else if (prev_alloc && !next_alloc) { ….. } else if (!prev_alloc && next_alloc) { size += GET_SIZE(HDRP(PREV_BLKP(bp))); PUT(FTRP(bp), PACK(size, 0)); PUT(HDRP(PREV_BLKP(bp)), PACK(size, 0)); bp = PREV_BLKP(bp); } else { ……. } return bp; }

18. Adding Debugging Information • mm_heapcheck • Display “request_id” and “payload” of every active block • request_id : malloc request counter (0.. • mm_init sets the counter to 0 • mm_malloc increments the counter • payload size : the memory requested by malloc • Can be different from the allocated size!!! • We need to store the info somewhere ?

19. Allocated Block Format Header 32 bits 32 bits request_id payload size 32 bits Pointer returned to user >= what user asked for in malloc Payload Footer 32 bits

20. One Way to Implement This • Inside malloc • Allocate additional memory in malloc PUT(bp,request_counter); PUT(bp+4,size); return bp+DSIZE; • Inside Free • bp = bp – DSIZE;

21. Heapcheck • Put all sorts of sanity checks • Scan the implicit list • like the first fit function • print request_id and size

22. Explicit Lists • Separate Free List • Can find a free block quickly • Change Free Block Format • Add prev pointer • Add next pointer • Where to store free list pointer • Only one WORD • Can store in unused PAD word • Some functions to add • static void insertfree_block(void * freeblkptr); • static void removefree_block(void * freeblkptr);